1. Field of the Invention
This invention relates to separation apparatus used for separating materials such as solid particles (e.g. dust and dirt) from a fluid (e.g. air or water) or one liquid (e.g. oil) from another (e.g. produced water). The mixture of materials to be separated is fed tangentially at a high rate into an inlet of at least one cyclone incorporated in the apparatus, and after separation passes out through one of two outlets known as the underflow outlet and overflow outlet respectively.
2. Discussion of Prior Art
Naval Technical Bulletin Disclosure Volume 4 No 12 December 1979 at pages 21-25 discloses a centrifugal fuel filtering device including a cyclone into which supply fuel is injected at a regulated tangential velocity by a differential pressure-responsive piston. A vortex is established within the cyclone thereby concentrating heavier contaminated fuel at the outer cyclone radius and lighter clean fuel at an inner radius about a screen through which the clean fuel is extracted.
It is known that when fluid flows through a cyclone, the pressure differential across the cyclone is related to the flow rate by the approximate equation EQU P.about.Kq.sup.n ( 1)
where
P=pressure differential PA1 q=flow rate PA1 K=a constant PA1 n=a number between 2 and 3
K and n are dependent on the geometry of the cyclone. It can be seen therefore that increasing the flow rate q causes an increased pressure differential P.
A cyclone is normally operated so that the pressure differential from the inlet to the overflow outlet is the same as, or is a constant ratio of, the pressure differential from the inlet to the underflow outlet. In cyclones for separating solids from liquids these pressure differentials are normally equal, and the pressure differentials are maintained by discharging material through both the underflow and overflow outlets to atmospheric pressure. In cyclones for separating oil from produced water the pressure differential to the overflow outlet is typically twice the pressure differential to the underflow outlet and, because in this type of application the fluid flows often fluctuate, these pressure ratios are usually maintained by closed loop control systems.
It is generally desirable to operate a cyclone such that the pressure differential across the cyclone does not fall below a certain value (because for example efficiency may be reduced), or exceed a certain value (because for example wear rate becomes higher, pumping costs become higher, available pressure is exceeded, or in the case of liquid/liquid cyclones the efficiency may be reduced). For any given cyclone and application the pressure differential across the cyclone is related to the flow rate as indicated in the approximate equation (1) above. Other parameters do affect the pressure differential across the cyclone but their effects are generally insignificant in comparison to the effects of changes in flow rate. The desirable maximum and minimum operating pressure differentials can thus be equated to a maximum and minimum desirable flow rate per cyclone.
In order to keep a plurality of cyclones arranged in series or in parallel operating efficiently it is necessary to ensure that the flow rate through the or each cyclone is kept within a desirable range. Thus in a system which comprises a plurality of similar cyclones and which is subject to a varying flow rate, when the flow rate per cyclone exceeds the maximum desirable value the flow must be divided through a larger number of cyclones and when the flow rate per cyclone is less than the minimal desirable value the flow must be divided through a smaller number of cyclones. If each of said cyclones has a valve fitted to one or more of its ports, then increasing or decreasing the number of cyclones through which the flow is divided is simply a matter of operating the appropriate valves, and this may be accomplished without stopping or limiting the flow. If said cyclones are contained within a pressure vessel and are not individually valved, then either a plurality of vessels each containing different numbers of cyclones is necessary, combinations of these vessels being used over a certain fraction of the total flow range, or the vessel must be opened and the number of operating cyclones it contains altered. If stoppages in the flow cannot be accepted, then to alter the number of cyclones in a vessel will require a second vessel through which the flow can be passed while the number of cyclones in the first vessel is altered.